Fig. 4.

SpA influences the stability of antibodies in vivo. (A and B) Human and mouse hFcRn/mFcRn binding to human (A) and mouse (B) IgG at pH 6.0 was measured in the presence of PBS, SpA, or SpA_KKAA by ELISA (n = 3 assays). ELISA plates were coated with hIgG, mIgG, or mock (PBS). Bound biotinylated FcRn was detected with HRP-conjugated streptavidin, and absorbances were recorded at 450 nm (A₄₅₀). (C–D) Fold change over time of the serum concentration of mIgG2a and mIgG3 following injection of PBS, SpA, or SpA_KKAA (C) and infection with mock (PBS), Newman, or its spa_KKAA variant in BALB/c mice (D) (n = 4 animals per group). (E) hIgG1 and hIgG3 concentrations over time in μMT mice infected with mock (PBS), Newman, or spa_KKAA 12 h after administration of hIgG1/3 (n = 4 mice per group). (F and G) Serum concentrations over time of test antibodies following injection of PBS, SpA, or SpA_KKAA in BALB/c mice (n = 4 mice per group). (H) hFcRn binding to Tefi at pH 6.0 was measured in the presence of PBS, SpA, or SpA_KKAA by ELISA (n = 3 assays). Bound FcRn was detected as in A. (I) Serum concentrations over time of test antibodies in BALB/c mice (n = 5 mice per group). Animals were infected with the Newman variants Δclfa-spa⁺ or Δclfa-spa_KKAA or PBS (mock) treated 12 h after the administration of test antibodies. (J) ICs of test antibodies Tefi/Tefi^R and cognate antigen ClfA-A were injected in BALB/c mice (n = 5 mice per group) along with PBS, SpA, or SpA_KKAA, and the remaining fraction of test antibodies was measured over time. Data are presented as mean ± SEM and representative of at least two independent experiments (A, B, and H). Significant differences were identified by two-way ANOVA with Bonferroni posttests (**P < 0.01; *P < 0.05).